This invention relates to the flow measurement of solids that are fed into systems where such solids are conveyed by gas-entrainment. Gas-entrained solids flow is currently widely used in the mineral and chemical processing industry for conveying granular solids and powders within a processing plant. In proposed applications such as coal gasification or liquefaction, crushed coal can be fed to a reactor where it is converted to process gas in the first stage of the process. The yield of this conversion depends upon maintaining a stoichiometrical balance between reagents. Although the gaseous reagents are readily controlled and their feedrate is readily measured by known methods, the feedrate of gas-entrained solids is very difficult to measure at all and cannot be measured with a high degree of accuracy by present commercially available methods.
In a typical pneumatic conveyor, solids are injected into the gas stream through a mixing tee after descending vertically from a feedhopper that is restocked either intermittently or semi-continuously. A rough estimate of the solids flowrate can be obtained in the form of a load cell support apparatus associated with a feedhopper. The measurement obtained in this manner is at best intermittent. A continuous measurement is required for control purposes. Therefore, for gas-entrained conveyance of solids, there exists a need for continuous feedrate measurement of the solids.
Such a continuous measurement could in principle be effected within the conveyor duct, downstream from the solids injection point (mixing tee). However, it must be considered that the solids, upon entering the conveyor duct, undergo continuous acceleration while their concentration diminishes commensurately; the entraining gas also speeds up and the pressure drops over a considerable length of duct. To provide a determination of the solid mass flowrate or feedrate under these conditions therefore requires two measurements simultaneously and at the same duct location, e.g. a density and velocity measurement.
The highly erosive nature of the medium (in which granular solids are propelled at speeds up to 20 m/sec) favors non-intrusive methods for both these measurements. Certain non-intrusive techniques, specifically, capacitive on-line densitometry and cross-correlation processing of signals from a pair of capacitors, have been tried. However, at the present, neither of these measurement techniques can be provided at the desirable level of accuracy (in particular, the cross-correlation velocity readout tends to have a considerable error). Yet another problem, inherent in all hostile media and particularly burdensome for a dual reading scheme, is the difficulty of providing calibration in a situation where no other and more accurate scheme for local velocity/density measurement can be provided at a reasonable cost.
These considerations indicate that, instead of choosing a measurement site downstream from injection, a measurement be made in the vertical duct (downcomer) through which solids enter the gas steam. Here, a number of favorable factors can be exploited:
(1) The conditions, described above, that mandate a circular cross section in the conveyor duct are absent, hence, a rectangular geometry can be chosen. Such a geometry offers an exceptionally uniform sensing field, uniquely associated with a "flat-plate" capacitor. Consequently, the local density measurement can be made with especially high accuracy.
(2) Means can be provided to release the solids in such a way as to allow a close-to-free fall at high dilution. Under those conditions, the velocity at the mid-plane through the density measurement capacitor can be calculated, with only a small release-factor correction, and thus does not have to be measured.
(3) At relatively high dilution (small solid volume fraction), a highly accurate formula, needed to convert the measured bulk dielectric constant to solid volume fraction, can be provided.
(4) Combining (a) an accurate reading of the local bulk dielectric constant, (b) the accurately calculated particle velocity, and (c) known constants such as the dielectric constant and intrinsic density of the solid and the cross sectional area of the duct, yields a rapid as well as accurate determination of the solids feedrate which, except for fluctuations about an average, must remain constant throughout the conveyor duct.
Another advantage of a measurement site in the vertical duct is its potential for direct, interactive control of the solids feedrate. Such control requires a travel time (delay) between measurement and eventual process vessel injection to compensate for the inevitable lag between measurement reference time and delivery of the computed readout, use of the readout in an interactive control algorithm, and, finally action to reset the valve.
Still other specific advantages of combining capacitive sensing in the vertical feed duct are: operation in a relatively benign medium, hence, absence of problems arising from severe erosion; relatively inexpensive hardware as well as electronics, and ready access to the equipment for maintenance, etc.
Accordingly, it is an object of the present invention to provide an apparatus and method to continuously measure the solids feedrate in a gas entrained solids conveyance system (pneumatic conveyor).
It is a further object of this invention to provide an apparatus and method for solids feedrate measurement that requires no calibration.
It is another object of this invention to provide an apparatus and method for local bulk density measurement that affords a uniform and unbiased sensing of the entire duct cross section.
It is another object of this invention is to provide a means for measurement of the solids feedrate in a pneumatic conveyor system that does not require intrusive devices or probes.
It is yet another object of this invention to provide a measurement site that affords interactive control of the feedrate.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.